It is increasingly appreciated that many institutionalized elderly in the United States suffer from osteomalacia due to vitamin D deficiency. This disease could contribute to increased frequency of fractures of spine, wrist and hip which confer an enormous cost to the public in both acute and chronic nursing care. Despite a general acceptance of the problem, the exact incidence of vitamin D deficiency in the elderly has not been established nor is its etiology understood. The goals of the proposed studies are two fold. First, we intend to define the extend of vitamin D deficiency in groups of chronically institutionalized, as well as healthy ambulatory elderly by measurement of basal serum concentrations of vitamin D metabolites (25-hydroxyvitamin D and 1,25-dihydroxyvitamin D) and parathyroid hormone. Second, we will determine whether there are age-related reductions in the ability of the intestine to absorb vitamin D. This will be approached by comparing the kinetics of the appearance and disappearance of an oral dose of vitamin D in elderly individuals to results obtained in young, healthy adults. Information obtained from these studies should help to identify individuals at risk of developing osteomalacia and provide a foundation for the further study of the etiology and pathophysiology of vitamin D deficiency and osteomalacia in the aged.